The invention pertains to a projection display that projects an image produced by liquid-crystal light valves.
Conventional full-color projection displays using reflective light valves, such as that of Unexamined Japanese Patent Document 63-39294 are known. FIG. 13 shows an arrangement of such a conventional projection display. A white illumination-light flux is emitted from a light source 223 that comprises, for example, a halogen lamp. The illumination-light flux typically passes through a collimating lens 222 operable to make parallel the rays comprising the illumination-light flux. The illumination-light flux then enters a polarizing beamsplitter (PBS) 221 disposed along the optical axis O of a color-separation optical system 211.
S-polarized light of the illumination-light flux is reflected by the PBS 221 and is incident on the color-separation optical system 211. The s-polarized illumination-light flux incident on the color-separation optical system 211 is separated into the three primary colors, red (R), blue (B), and green (G), as follows.
The color separation optical system 211 includes a first prism 211A, a second prism 211B, and a third prism 211C, each disposed as shown in FIG. 13. A surface 211e of the first prism 211A is coated with a dichroic film that reflects blue light but transmits light with longer wavelengths (i.e., red and green light). There is a gap between the first prism 211A and the second prism 211B. A dichroic film that reflects red light but transmits green light is coated on a surface 211f of the second prism 211B, between the second prism 211B and the third prism 211C.
As the illumination-light flux reflected from the PBS 221 enters through an incidence surface 211a of the first prism 211A, blue light is reflected by the surface 211e and is then reflected inwardly by the surface 211a toward an emergence surface 211b of the first prism 211A. Red light that passes through the surface 211e of the first prism 211A is reflected by the surface 211f and is then reflected inwardly by the surface of the second prism 211B between the first and the second prisms. The inwardly reflected red light then exits through an emergence surface 211c of the second prism 211B. Green light that passes through the surface 211e of the first prism 211A and through the surface 211f of the second prism 211B travels toward an emergence surface 211d of the third prism 211C.
Reference numerals 212, 213, and 214 denote two-dimensional reflection-type liquid crystal light valves (LCLVs) for displaying a blue light image, a red light image, and a green light image, respectively. Each of the reflective-type LCLVs has a dielectric reflecting layer 215, 216, and 217, respectively, formed on the back of a respective transmission-type LCLV so that the LCLVs 215, 216, 217 operate as reflection-type LCLVs. As each color of light enters a respective LCLV, the light is modulated by the respective LCLV. Hence, each color""s video signal is converted into an image that has a transmission-rate distribution at the respective LCLV.
The modulated color light is then reflected and changed in polarization state by 90xc2x0. That is, the s-polarized light is converted by the LCLV to p-polarized light. The modulated and reflected color lights travel along reverse paths through the first, second, and third prisms 211A, 211B, 211C, respectively, to be combined into a single light flux. The resultant combined, single light flux emerges from the incidence surface 211a of the first prism 211A. The light flux whose polarization state has been converted is transmitted through the PBS 221 and projected on a screen 225 by a projection lens 224.
A problem with the conventional example shown in FIG. 13 is its inability to provide sufficiently high-contrast projected images. The conventional projection display described herein does not project an ideal xe2x80x9cblackxe2x80x9d image on the screen for the following reasons.
As linearly polarized light fluxes are incident on the dichroic films, after being passed through the PBS 221, the light flux is in part transmitted and in part reflected by the dichroic films. A light flux incident on a dichroic film and having a plane of polarization that is not entirely s-polarized or p-polarized with respect to the dichroic film is separated into s-polarized light and p-polarized light by the dichroic film. In addition, reflection and transmission by the dichroic film impose a phase difference between the s-polarized light and p-polarized light. As a result, the light flux exiting the dichroic film is typically elliptically polarized. Hence, the light flux transmitted by the PBS 221 includes light of undesirable polarization. The PBS 221 then directs the undesirable polarized light toward the screen 225. Accordingly, an ideal black image is not projected on the screen 225 and image contrast is degraded.
The light flux from the light source 223 is split into polarized components by the polarizing beamsplitter 221, and one of the polarized components is subsequently color-separated and color-combined by the prisms 211A, 211B, 211C. The polarizing beamsplitter 221 analyzes the color-combined light flux that is directed to the screen 225. A rotation of the plane of polarization of the light flux at the prisms 211A, 211B, 211C results in a degradation of image contrast as well. In order to prevent such a rotation of the plane of polarization, it is necessary to make the prisms 211A, 211B, 211C from a material having an extremely low birefringence, increasing material and fabrication costs. Even when low-birefringence materials are used, birefringence is not completely eliminated, and image contrast is degraded.
The invention provides projection displays that reduce image contrast deterioration caused by polarization changes in color-separation and color-combining optical systems. Furthermore, the invention provides projection displays that do not exhibit image-contrast degradation caused by birefringence in the color-separation and color-combining optical systems.
Projection displays according to the invention preferably comprise a color-separation optical system having a plurality of substantially parallel dichroic mirrors. The color-separation system separates a light flux from a light source into multiple (e.g., first, second, and third) color components. Alternatively, the dichroic mirrors of the color-separation system are arranged to form a crossed dichroic-mirror or prism.
A separate light valve is provided for individually modulating each corresponding color component. Multiple (e.g., first, second, and third) polarizing beamsplitters are provided to polarize the color components before the color components are incident to corresponding light valves; the polarizing beamsplitters further serve to analyze the color components after modulation and reflection by the light valves. Because each polarizing beamsplitter is used with a single color component, the polarizing beamsplitters can have performance superior to that of a polarizing beamsplitter to be used with multiple color components.
A color-combining optical system is provided to re-combine the color components after the color components are modulated and analyzed. The color-combining optical system preferably comprises an L-shaped dichroic prism having a plurality of substantially parallel dichroic reflecting surfaces. Alternatively, a plurality of substantially parallel dichroic mirrors can be provided or a plurality of substantially parallel dichroic films. Similarly, crossed-dichroic mirrors can be used instead of a crossed-dichroic prism. Because the color-combining system receives the color components after analysis by the polarizing beamsplitters, birefringence and other polarization effects in the color-combining system have little effect on image contrast. Expensive, low-birefringence materials and mountings are unnecessary.
A projection optical system receives the combined, modulated, and analyzed color components from the color-combining optical system and forms an image.
Each color component preferably passes through a corresponding field lens placed between the color-separation optical system and the corresponding light valve. Such field lenses collimate respective color components before reaching the light valves, thereby reducing the range of angles of incidence of the color components on the light valves. The field lenses preferably cause the chief rays of the projection lens, i.e. rays that pass through the center of the aperture stop of the projection lens, to propagate parallel to an optical axis when incident on the polarizing beamsplitters, the color-combining system, and the light valves. The field lenses direct the color components so that chief rays incident on the light valves are parallel throughout the modulation regions of the light valves. This arrangement reduces contrast variation and color shading caused by angle-of-incidence variations in the modulation of the light valves. Such an arrangement also reduces image-contrast degradation due to angle-of-incidence dependencies in the polarizing beamsplitters and the color-combining system. In particular, color shading in a projected image due to angle-of-incidence variations of the color-combining system is reduced.
The projection displays preferably comprise a telecentric projection lens. In a telecentric lens, a chief ray (i.e., a ray passing through the center of the aperture stop) propagates parallel to an axis after passing though the lens.
Optical path lengths from the illumination system to the light valves are preferably equal for all color components. Similarly, optical path lengths from the light valves to the projection lens are preferably equal. Alternatively, optical path lengths for two of the color components are equal and a relay optical system compensates for the differing optical path of the third.
The illumination system preferably directs a light flux to an integrator such as a rod integrator. A relay lens and a field lens are preferably provided that image an exit surface of the integrator onto the light valves. In conjunction with the telecentric projection lens and field lenses associated with each light valve, telecentric critical illumination is achieved.
The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description of Example Embodiments which proceeds with reference to the accompanying drawings.